1. Field of the Invention
The invention relates to a method of treating iron in a silver recovery process, and more particularly a method of preventing the formation of iron sludge or precipitate in a silver recovery process, such as is used on standard photographic solutions.
2. Related Art
There are many chemical processes, particularly those related to photographic processing, that result in waste fluids that contain a significant amount of dissolved silver. As silver is a semiprecious metal, it is generally economically beneficial to remove the silver from the waste fluid prior to disposal. In addition, silver is a regulated material. Consequently, there are limits to the amount of silver that can be present in fluids that are discharged into sewer systems. As a result, there is both an economic and environmental incentive to recover the dissolved silver from these waste fluids.
There are a number of silver recovery methods that are known to those of skill in the art. In the photographic processing field, the recovery process often involves a metal recovery canister in which a metal replacement medium, such as steel wool, is contained. The chemical fluid, such as a standard photochemical solution containing silver, is passed through the metal recovery canister and then sent via a drain to the sewer system. The recovery process is an ion exchange process that releases iron out of the container as the silver is recovered. The silver forms a black sludge-like precipitate that remains in the canister. When the canister has either processes a certain amount of photochemical solution or once a certain amount of silver has been recovered, the canister is replaced with a fresh canister. One example of such a metal recovery canister is described as well as a discussion of others recovery methods in U.S. Pat. No. 6,096,209, the contents of which are hereby incorporated herein for its discussion of the background, criteria and discussion of various silver recovery technologies.
One issue regarding the use of metal recovery canisters is that, while the silver is removed from the chemical fluid, the iron that is released will form a sludge or precipitate in the drain lines and other components down stream from the metal recovery canister. One way of handling this problem has been to use a mechanical router to remove the iron from the drain line periodically, such as once a year. Another way of handling it has been to frequently rinse the drain line to prevent the iron sludge or precipitate from building up. Some companies have sold a dry powder that when mixed with water creates an acidic slurry that redissolves the iron compounds and keeps them in solution. One such dry powder is available from Academy Corporation of Albuquerque N. Mex. under the name Rustbuster.® However, most users of metal recovery canisters are not willing to use an acid slurry. A couple of years ago there was also a product introduced to the market that kept the iron in solution. Unfortunately, the U.S. Environmental Protection Agency (EPA) deemed the material to be hazardous. Therefore, there is still a need for a method to prevent the iron deposits in the drain line that uses a non-hazardous material and does not require an acid slurry.